# Working mass > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Working_mass > Markdown URL: https://mediated.wiki/source/Working_mass.md > Source: https://en.wikipedia.org/wiki/Working_mass > Source revision: 1302418121 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Mass against which a system operates}} '''Working mass''', also referred to as '''reaction mass''', is a [mass](/source/mass) against which a system operates in order to produce [acceleration](/source/acceleration). In the case of a chemical rocket, for example, the reaction mass is the [product](/source/Product_(chemistry)) of the burned fuel shot backwards to provide propulsion. All acceleration requires an exchange of [momentum](/source/momentum), which can be thought of as the "unit of movement". Momentum is related to mass and velocity, as given by the formula ''P = mv,'' where ''P'' is the momentum, ''m'' the mass, and ''v'' the velocity. The velocity of a body is easily changeable, but in most cases the mass is not, which makes it important. The working mass of a fuel can be determined by mathematical calculation.{{cite book | title=Fuels and Petroleum Processing | publisher=Krishna Prakashan Media | url=https://www.google.co.uk/books/edition/Fuels_and_Petroleum_Processing/71yyE2kaZg4C?hl=en&gbpv=1&dq=%2522Working+mass%2522+-wikipedia&pg=PA5&printsec=frontcover | access-date=2025-07-25 | page=F5}} ==Rockets and rocket-like reaction engines== In rockets, the total velocity change can be calculated (using the [Tsiolkovsky rocket equation](/source/Tsiolkovsky_rocket_equation)){{cite book | last=Skiba | first=Richard | title=Rocket Design and Construction Fundamentals | publisher=After Midnight Publishing | date=2024-12-20 | isbn=978-1-76380-466-1}}{{cite book | last=Fayngold | first=Moses | title=Special Relativity and How it Works | publisher=John Wiley & Sons | publication-place=Weinheim | date=2008-07-21 | isbn=978-3-527-40607-4 | page=572}} as follows: \Delta\,v = u\,\ln\left(\frac{m + M}{M}\right) Where: * ''v'' = ship velocity. * ''u'' = exhaust velocity. * ''M'' = ship mass, not including the working mass. * ''m'' = total mass ejected from the ship (working mass). The terms working mass or reaction mass are used primarily in the [aerospace](/source/aerospace), [aeronautics](/source/aeronautics) and [astronautics](/source/astronautics) fields.{{cite book | last=Jenkins | first=C. H. | title=Progress In Astronautics and Aeronautics: Gossamer Spacecraft: Membrane and Inflatable Structures Technology for Space Applications | publisher=AIAA | date=2001 | isbn=978-1-60086-442-1 | url=https://www.google.co.uk/books/edition/Progress_In_Astronautics_and_Aeronautics/NZahLD6gXcoC?hl=en&gbpv=1&dq=reaction+mass+%252B+aerospace&pg=PA482&printsec=frontcover | access-date=2025-07-25 | page=482}} In more "down to earth" examples, the working mass is typically provided by the Earth, which contains so much momentum in comparison to most vehicles that the amount it gains or loses can be ignored. However, in the case of an [aircraft](/source/aircraft) the working mass is the air, and in the case of a [rocket](/source/rocket), it is the rocket fuel itself. Most rocket engines use light-weight fuels (liquid [hydrogen](/source/hydrogen), [oxygen](/source/oxygen), or [kerosene](/source/kerosene)) accelerated to supersonic speeds. However, [ion engine](/source/ion_engine)s often use heavier elements like [xenon](/source/xenon) as the reaction mass, accelerated to much higher speeds using electric fields. In many cases, the working mass is separate from the [energy](/source/energy) used to accelerate it. In a car, the engine provides power to the wheels, which then accelerates the Earth backward to make the car move forward. This is not the case for most rockets, however, where the rocket propellant is the working mass, as well as the energy source. This means that rockets stop accelerating as soon as they run out of fuel, regardless of other power sources they may have. This can be a problem for satellites that need to be repositioned often, as it limits their useful life. In general, the exhaust velocity should be close to the ship velocity for optimum [energy efficiency](/source/Efficient_energy_use). This limitation of rocket propulsion is one of the main motivations for the ongoing interest in [field propulsion](/source/field_propulsion) technology. ==References== {{Reflist}} ==See also== * [Rocket equation](/source/Rocket_equation) {{DEFAULTSORT:Working Mass}} Category:Aerospace engineering Category:Mass --- Adapted from the Wikipedia article [Working mass](https://en.wikipedia.org/wiki/Working_mass) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Working_mass?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.